The present invention relates to a warning apparatus for detecting the presence or the absence of an intruding object by using microwaves.
There has been provided a warning apparatus with the combination of a transmitter and a receiver by using light beam, an ultrasonic wave or a microwave, in which when an intruding object appears between the transmitter and the receiver, a receiving signal level changes. Depending on the change of the receiving signal level, the warning apparatus recognizes that the intruding object is present. Of those warning apparatuses, the warning apparatus using the microwave has been widely used particularly in an important location, for the reason that it suffers from little erroneous detection with a high reliability, compared to those using the light beam and the ultrasonic wave. A detecting area of the warning apparatus is mainly determined by a shape of the microwave beam transmitted. In designing the detecting area of the apparatus, it is preferable to form so called electromagnetic fence in which it is wider in a vertical plane so as to prevent an intruder from passing over or under the microwave beam defining the detecting area and it is narrower in a horizontal plane so as to prevent obstructive object therearound such as trees sprayed by wind or passengers from entering the detecting area. Therefore, a desirable beam to be transmitted is an elongated fan beam. The conventional antenna commonly used to radiate the fan beam is a cut parabolic reflector antenna using a part of a parabolic reflector or a sectorial horn antenna. To transmit the desired fan beam, whose band width is over 30.degree. in a vertical plane and below 8.degree. in a horizontal plane, the antenna is necessarily enlarged in size, however. In the case of the parabolic antenna at 10 GHz, for example, the physical dimensions required are 30 cm for the diameter, and 20 cm for the radiator. Consequently, the radiation efficiency decreases. Also, in the case of the sectorial horn antenna, 30 cm or more is required for the length of the horn. In both cases, for this reason, those antennas are restricted in the location where they are installed. In connection with this, a large antenna is distinguished, and hence the nature, i.e. inconspicuousness, essentially required for the warning apparatus is lost of such an antenna is used.